Time-of-flight mass spectrometers with cassini reflector

ABSTRACT

The invention relates to embodiments of high-resolution time-of-flight (TOF) mass spectrometers with special reflectors. The invention provides reflectors with ideal energy and solid angle focusing, based on Cassini ion traps, and proposes that a section of the flight path of the TOF mass spectrometers takes the form of a Cassini reflector. It is particularly favorable to make the ions fly through this Cassini reflector in a TOF mass spectrometer at relatively low energies, with kinetic energies of below one or two kiloelectronvolts. This results in a long, mass-dispersive passage time in addition to the time of flight of the other flight paths, without increasing the energy spread, angular spread or temporal distribution width of ions of the same mass. It is also possible to place several Cassini reflectors in series in order to extend the mass-dispersive time of flight. Several TOF mass spectrometers for axial as well as orthogonal ion injection with Cassini reflectors are presented.

FIELD OF INVENTION

The invention relates to time-of-flight mass spectrometers withspecially shaped reflectors.

BACKGROUND

In the prior art, there are essentially two types of high-resolutionreflector time-of-flight mass spectrometers, which are characterizedaccording to the way the ions are injected.

Time-of-flight mass spectrometers with axial injection include MALDItime-of-flight mass spectrometers (MALDI-TOF MS), which operate withionization by matrix-assisted laser desorption, but also time-of-flightmass spectrometers where stored ions are injected axially into theflight path from a storage device such as an RF quadrupole ion trap.They usually have Mamyrin reflectors (B. A. Mamyrin et al., “Themass-reflectron, a new nonmagnetic time-of-flight mass spectrometer withhigh resolution”, Sov. Phys.-JETP, 1973: 37(1), 45-48) in order totemporally focus ions with an energy spread. Mamyrin reflectors allowsecond-order temporal focusing, but not higher order focusing. Sincepoint ion sources are used, the reflectors can be gridless, as amodification of the Mamyrin reflectors, which are operated with grids.MALDI-TOF MS are operated with a delayed acceleration of the ions in theadiabatically expanding laser plasma and with high accelerating voltagesof up to 30 kilovolts; in good embodiments, with a total flight path ofaround 2.5 meters, they achieve mass resolving powers of R=50 000 in amass range of around 1000 to 3000 daltons.

Time-of-flight mass spectrometers where a primary ion beam undergoespulsed acceleration at right angles to the original direction of flightof the ions are termed OTOF-MS (orthogonal time-of-flight massspectrometers). FIG. 1 depicts a simplified schematic of such anOTOF-MS. The mass analyzer of the OTOF-MS has a so-called ion pulser(12) at the beginning of the flight path (13), and this ion pulseraccelerates a section of the low-energy primary ion beam (11), i.e. astring-shaped ion packet, into the flight path (13) at right angles tothe previous direction of the beam. The usual accelerating voltages,only small fractions of which are switched at the pulser, amount tobetween 8 and 20 kilovolts. This forms a ribbon-shaped secondary ionbeam (14), which consists of individual, transverse, string-shaped ionpackets, each of which is comprised of ions having the same mass. Thestring-shaped ion packets with light ions fly quickly; those withheavier ions fly more slowly. The direction of flight of thisribbon-shaped secondary ion beam (14) is between the previous directionof the primary ion beam and the direction of acceleration at rightangles to this, because the ions retain their speed in the originaldirection of the primary ion beam (11). A time-of-flight massspectrometer of this type is also preferably operated with a Mamyrinenergy-focusing reflector (15), which reflects the whole width of theribbon-shaped secondary ion beam (14) with the string-shaped ionpackets, focuses its energy spread, and directs it toward a flatdetector (16). The width of the ion beam means the reflector must beoperated with grids. Mass resolving powers of around R=40 000 at mass1000 daltons are achieved in these OTOF mass spectrometers.

As these two examples suggest, time-of-flight mass spectrometers withhigh mass resolution are operated predominantly with Mamyrin reflectorsin today's technology. Mamyrin reflectors provide second-order energyfocusing, but not higher order focusing. If the energy spread of theions is relatively large compared to the average energy, undesirablefocusing errors occur. Since the kinetic energy of the ions alwaysspreads slightly as the ions are being produced, or during their pulsedacceleration, the time-of-flight mass spectrometers must be operatedwith high accelerating voltages for the ions, between 5 and 30kilovolts, for example, in order to always keep the relative energyspread as small as possible in relation to the average energy.

As a consequence of the high ion energy, the very long flight paths mustbe chosen in order to achieve a good temporal dispersion of ions ofdifferent masses. Since the fastest ion detectors at present offermeasurement rates up to five billion measurements per second, and thusrequire a separation of a few nanoseconds between two ion masses whichare to be resolved, the flight paths for the high mass resolutionsdesired must be several meters long, often far more than ten meters. Ifmultiple reflectors are used to keep the instrument compact and toextend the flight path, the residual errors of the reflectors add up. Iflower accelerating voltages are used in order to manage with shorterflight paths, the resulting higher relative energy spread, which cannotbe focused in a higher order, prevents a high resolving power from beingachieved.

It is known that a quadratically increasing electric potential in thereflector results in an ideal reflection with energy focusing of as highan order as desired (T. J. Cornish et al., “A curved field reflectrontime-of-flight mass spectrometer for the simultaneous focusing ofmetastable product ions”, Rapid Commun. Mass Spectrom., 1994: 8(9),781-785). If such a field is generated in a simple diaphragm stack byvoltages which increase quadratically from aperture to aperture, theresult is a defocusing effect in both lateral directions. If the kineticenergy of the ions is decreased in order to achieve long dispersivetimes of flight, the laterally defocusing effect increases. Furtherelectric fields for at least “quasi-ideal” energy focusing are presentedin a publication by A. A. Makarov, J. Phys. D; Appl. Phys. 24, 533(1991).

Kingdon ion traps are generally electrostatic ion traps in which ionscan orbit one or more inner electrodes or oscillate between severalinner electrodes. An outer, enclosing housing is at a DC potential whichthe ions with a predetermined total energy (sum of kinetic and potentialenergy) cannot reach. In special Kingdon ion traps which are suitablefor use as mass spectrometers, the inner surfaces of the housingelectrodes and the outer surfaces of the inner electrodes can bedesigned in such a way that, firstly, the motions of the ions in thelongitudinal direction of the Kingdon ion trap are completely decoupledfrom their motions in the transverse direction and, secondly, asymmetrical, parabolic potential profile is generated in thelongitudinal direction in which the ions can oscillate harmonically inthe longitudinal direction. When “Kingdon ion traps” are mentionedbelow, this always refers to these special designs.

In the publications DE 10 2007 024 858 A1 (C. Köster) and DE 10 2011 008713 A1 (C. Köster), Cassini ion traps are described as special types ofKingdon ion traps which differ in the way in which several innerelectrodes are arranged. The inner electrodes and the outer housingelectrode (and possibly several segmented housing electrodes also) aredesigned here in such a way that the longitudinal motion is completelydecoupled from the transverse motion, and a parabolic potential well isgenerated in the longitudinal direction for a harmonic oscillation.

The potential distribution φ(x, y, z) of such a Cassini ion trap can,for example, be that of a hyperlogarithmic field of the following form:

${\psi \left( {x,y,z} \right)} = {{{\ln\left\lbrack \frac{\left( {x^{2} + y^{2}} \right)^{2} - {2 \cdot b^{2} \cdot \left( {x^{2} - y^{2}} \right)} + b^{4}}{{ai}^{4}} \right\rbrack} \cdot \frac{U_{l\; n}}{C_{l\; n}}} + {\left\lbrack {{{- \left( {1 - B} \right)} \cdot x^{2}} - {B \cdot y^{2}} + z^{2}} \right\rbrack \cdot \frac{U_{quad}}{C_{quad}}} + U_{off}}$

The shape of the field can be changed by the constants a, b and B.U_(ln), U_(quad) and U_(off) are potential voltages. The inner surfaceof the outer housing and the outer surfaces of the inner electrodes areequipotential surfaces φ(x,y,z)=const. of this potential distribution.In cross-section, the equipotential lines form approximate Cassini ovalsabout the inner electrodes here; two inner electrodes result in Cassiniovals of the second order, while n inner electrodes result in Cassiniovals of the nth order. For an even number of inner electrodes, thereare embodiments where the ions can oscillate transversely near thecenter plane between at least one pair of inner electrodes. Any ratio ofthe longitudinal oscillation period to the transverse oscillation periodcan be set with the aid of form parameters.

In view of the foregoing, there is a need to provide compacttime-of-flight mass spectrometers with high mass resolution, andespecially to provide reflectors for time-of-flight mass spectrometerswhose energy and solid angle focusing are as ideal as possible.

BRIEF SUMMARY OF THE INVENTION

The present invention provides a time-of-flight mass spectrometer withan ion source, a flight path and an ion detector, wherein at least asection of the flight path of the time-of-flight mass spectrometer has apotential distribution of a Cassini ion trap with several innerelectrodes, preferably an even number of electrodes, the Cassini iontrap being shaped for decoupled oscillations of the ions in thelongitudinal and the lateral directions.

A time-of-flight mass spectrometer according to the invention preferablyhas at least one field-free section of the flight path and at least onereflector with the potential distribution of a Cassini ion trap withseveral inner electrodes shaped for decoupled oscillations of the ionsin the longitudinal and the lateral directions. The at least onereflector can, for example, comprise a halved Cassini ion trap with ahousing, two inner electrodes and a terminating equipotential plate withelectrodes, where the electrodes of the equipotential plate trace theequipotential surfaces of the potential distribution of the Cassini iontrap at the location of the equipotential plate. The equipotential platehere has apertures for the injection and ejection of ions, while theshape of the reflector and the positions of the injection and ejectionapertures are preferably designed so that ions with the same mass passthrough an odd whole number of transverse half oscillations in thereflector. The housing of a Cassini reflector can also be constructed asa stack of apertured diaphragms, especially of identically shapedapertured diaphragms, connected to a voltage supply which generates apotential that increases quadratically from diaphragm to diaphragm.

In a time-of-flight mass spectrometer according to the invention, agreater part of the flight path of the time-of-flight mass spectrometercan have a potential distribution of a Cassini ion trap with severalinner electrodes, the Cassini trap shaped for decoupled oscillations ofthe ions in the longitudinal and the lateral directions, i.e. ionsexperience a potential distribution of a Cassini ion trap over more thanhalf of the flight path in the time-of-flight mass spectrometer (or inthe mass-dispersive section of the time-of-flight mass spectrometer).This greater part preferably comprises one or more halved Cassini iontraps, each having two inner electrodes and at least one terminatingequipotential plate.

A time-of-flight mass spectrometer according to the invention can haveat least one diaphragm system (acceleration and/or deceleration unit forions), which shapes the kinetic energy of the ions in such a way thatthe ions pass through the Cassini reflector, or through the flight pathwith the potential distribution of a Cassini ion trap, with a kineticenergy of around ten kiloelectronvolts at most, preferably less than twokiloelectronvolts, in particular less than one kiloelectronvolt.Furthermore, the time-of-flight mass spectrometer may include an RFquadrupole ion trap or a puller for the orthogonal injection of an ionbeam. The ion source of the time-of-flight mass spectrometer can be aMALDI ion source, for example, but electrospray ion sources or othertypes of ionization, especially in combination with orthogonalinjection, are also possible. The ion detector is preferably an iondetector with a secondary electron multiplier, but can also be a Faradaydetector. The ion detector here is arranged in such a way with respectto the flight path of the ions that the ions are destroyed on arrival atthe ion detector. In particular, the exit of a Cassini reflector can beequipped with an ion acceleration system with a conversion plate forconverting ions into electrons, which then fly backwards through theCassini reflector; and a secondary electron multiplier which detects theelectrons is mounted behind an equipotential plate at the rear.

The invention provides reflectors with ideal focusing, which are basedon Cassini ion traps, and proposes that a section of the flight path ofa time-of-flight mass spectrometer takes the form of a Cassinireflector. Cassini reflectors can focus ions of the same mass in anideal way according to energy as well as solid angle of injection. It isparticularly favorable to make the ions fly through this Cassinireflector in a time-of-flight mass spectrometer at relatively lowenergies, with kinetic energies of below one or two kiloelectronvolts.This results in a long mass-dispersive passage time in addition to thetime of flight of the other flight paths, without increasing the energyspread, angular spread or temporal distribution width of ions of thesame mass. It is also possible to place several Cassini reflectors inseries in order to extend the mass-dispersive time of flight. Thevoltages at the electrodes (apertured diaphragms or electrodes shapedaccording to the potential distribution) of a Cassini reflector or aCassini flight path can be provided by one or more capacitors or byseveral electro-chemical batteries (especially rechargeable batteries).

BRIEF DESCRIPTION OF THE ILLUSTRATIONS

FIG. 1 shows a schematically simplified representation of atime-of-flight mass spectrometer which corresponds to the prior art.Ions are generated at atmospheric pressure in an ion source (1) with aspray capillary (2), and these ions are introduced into the vacuumsystem through a capillary (3). A conventional RF ion funnel (4) guidesthe ions into a first RF quadrupole rod system (5), which can beoperated as a simple ion guide, but also as a mass filter for selectinga species of parent ion to be fragmented. The unselected or selectedions are fed continuously through the ring diaphragm (6) and into thestorage device (7); selected parent ions can be fragmented in thisprocess by energetic collisions. The storage device (7) has a gastightcasing and is charged with collision gas through the gas feeder (8) inorder to focus the ions by means of collisions and to collect them inthe axis. Ions are extracted from the storage device (7) through theswitchable extraction lens (9); this lens together with the einzel lens(10) shapes the ions to a fine primary beam (11) and sends them to theion pulser (12). The ion pulser (12) pulses out a section of the primaryion beam (11) orthogonally into the high-potential drift region (13),which is the mass-dispersive region of the time-of-flight massspectrometer, thus generating the new ion beam (14). The ion beam (14)is reflected in the reflector (15) with second-order energy focusing,and measured in the detector (16). The mass spectrometer is evacuated bythe pumps (17), (18) and (19).

FIG. 2 shows a three-dimensional representation of an electrostaticKingdon ion trap of the Cassini type, according to C. Köster, with ahousing electrode which is transversely split in the center into twohalf-shells (20 and 21) and two spindle-shaped inner electrodes (23,24). The Kingdon ion trap can be filled with ions through an entrancetube (25); the ions then move on oscillational paths (26). This Kingdonion trap also corresponds to the prior art.

FIG. 3 is a schematic representation of three cross-sections through aCassini ion trap whose outer housing (30) and inner electrodes (31) aredesigned so that the oscillation periods in the lateral direction and inthe longitudinal direction are equal. The ions can therefore fly alongsimple, closed trajectories (32, 33) and be ideally focused according toenergy and solid angle in both the upper and the lower summit.

FIG. 4 depicts a Cassini ion trap according to FIG. 3, which can be usedaccording to the invention as a reflector. The right half of the housing(35) is slightly smaller than the left half, and is also supplied with aslightly lower voltage difference to the inner electrodes (31) so thatthe electric fields in the interior of the Kingdon ion trap aremaintained. If ions are injected at the injection point (36) with asuitable average energy, but with both solid angle spread and energyspread, they are transferred to the exit point (37) and ideally focusedin the process in terms of both solid angle and energy, and not just insecond order.

FIG. 5 provides a view into the interior of a Cassini reflector in theform of half a Cassini ion trap with housing (30) and two innerelectrodes (31). The Cassini reflector here is terminated by anequipotential plate (38), which carries line-shaped electrodes (39)applied to the interior of the Cassini reflector, which follow thecorresponding equipotential lines of the Kingdon ion trap and aresupplied with voltages in such a way that the original electric field ofthe Kingdon ion trap from FIG. 3 is restored. The line-shaped electrodes(39) applied to the equipotential plate (38) are shown here only in arough schematic way. They approximately follow the familiar second-orderCassini ovals about the two inner electrodes. Ions can be injected intothe interior of the Cassini ion trap through an introductory slit (36)in the equipotential plate (38). These ions will then leave againthrough the exit slit (37), focused in an ideal way in terms of solidangle and energy. This reflector can also refocus ions with a broadenergy spread ideally, even if the ions fly with low energy and have arelatively high energy spread.

FIG. 6 depicts a time-of-flight mass spectrometer which uses threeCassini reflectors (46, 47, 48). An ion feed, not shown here, produces afine ion beam (40), which flies into the plane of the diagram and entersthe pulser (41). The fine ion beam (40) corresponds to the fine ion beam(11) in FIG. 1), and can be generated in a similar way. The pulser (41)now pulses out a small section of the ion beam (40) toward the firstCassini reflector (46). The angular offset of the ion beam (43) iscorrected by a deflection capacitor (42). The outpulsing can be done atlow energy, with conventional spatial and energy focusing according toWiley and McLaren, which has its focal point at the injection point (45)(“Time-of-Flight Mass Spectrometer with Improved Resolution”, W. C.Wiley and I. H. McLaren, Rev. Sci. Instrum., 26, 1150 (1955)). Thelow-energy ions are then guided in an ideal way through the Cassinireflectors (46), (47) and (48) and refocused according to energy andsolid angle at the exit (49). The ions can then be accelerated to highenergies of 10 to 30 kilovolts in the acceleration unit (50) andreflected in the reflector (51) in an energy-focusing way onto thedetector (52) within the housing (53), which is at a high voltage.

FIG. 7 represents a view into a Cassini reflector which is designed, andwhose injection and ejection openings are positioned, in such a way thatthe ions execute precisely one and a half transverse oscillations in thereflector during the longitudinal half oscillation. The equipotentialplate (95) with the printed electrodes obviates the need for the secondhalf-space and allows the ions to be injected and ejected through thisplate. The ions injected through the injection aperture (93) fly ontrajectories (92) and are focused in an ideal way onto the ejectionaperture (94), while ions of the same mass are focused in terms of timeand solid angle in relation to both their energy spread and theirangular spreads in both lateral directions. The greater penetrationdepth of the ions compared to the arrangement in FIG. 5 allows asignificantly wider relative spread of the ion injection energies thanthe arrangement according to FIG. 5.

FIG. 8 represents a time-of-flight mass spectrometer which firstcollects the ions in an RF quadrupole Paul ion trap and cools them toform a tiny cloud (62). The ions are fed to the ion trap with end capelectrodes (63, 65) and ring electrode (64) via an RF quadrupole ionguide (60) and an ion lens (61), and are cooled there by a damping gas.The ions can be mass selectively ejected in the usual way and measuredas a mass spectrum in a channeltron electron multiplier (66) via an ionelectron converter (67). But the ions of the ion cloud (62) can also besimultaneously accelerated and pulsed out into an essentially field-freeflight path (68), decelerated again in the diaphragm system (70), andfed to the Cassini reflector (72) at low energy with ideal angle andenergy focusing at the injection location (71). The Cassini reflector isterminated at both the front and the back with equipotential plates (75)and (74) respectively. The equipotential plates (75) and (74) are coatedwith fine conductive tracks, which reproduce the equipotential surfaces,and are supplied with the correct potentials in order to maintain theCassini potential. The ions leaving the exit aperture (77) arepost-accelerated in the diaphragm system (78) with 10 to 30 kilovoltsand reflected onto the ion detector (81) in the reflector (80) so as tofocus the energy.

FIG. 9 illustrates how ion beams are injected through apertures (112,114) in the equipotential plate (111) outside of the center plane, andleave again through apertures (113, 115), focused according to energyand solid angle, outside the center plane.

FIG. 10 shows how this behavior can be used for a double passage. Thebeam (105) enters through the equipotential plate (103), is reflectedbetween the two inner electrodes (100) of the first Cassini reflector,exits again, is reflected between two further inner electrodes (101) ofa second Cassini reflector, re-enters through the equipotential plate(103), is again reflected between the inner electrodes (100) of thefirst Cassini reflector, and re-emerges as a beam (106).

FIG. 11 shows a Cassini reflector of a different design but with thesame electric field: The outer housing here is replaced by a stack ofidentical apertured diaphragms (122). The apertured diaphragms haveinner openings in the form of a Cassini oval. In order to maintain theelectric field of a Cassini ion trap, the apertured diaphragms aresupplied with a quadratically increasing potential from the direction ofthe equipotential plate (120). The equipotential plates (120) and (121)correspond to those in FIG. 7. Ions of of the same mass but differentenergies fly on trajectories (124) which penetrate into the reflector todifferent depths but which all have precisely the same time of flight.

DETAILED DESCRIPTION

The invention provides reflectors with ideal energy and angle focusing,based on the electric fields in Cassini ion traps, and particularlyproposes that a section of the flight path of a time-of-flight massspectrometer takes the form of a Cassini reflector. It is particularlyfavorable to make the ions fly through this Cassini reflector atrelatively low energies, with kinetic energies as far below onekiloelectronvolt as possible. This results in a long mass-dispersivepassage time in addition to the time of flight of the other flightpaths, without increasing the energy spread ΔE, the angular spreadsΔφ_(x) and Δφ_(y) of the ions, or their temporal distribution width Δt,which they have acquired in the previous section of the flight path ofthe time-of-flight mass spectrometer. The time of flight of a singlycharged ion of mass 500 Da in one of the Cassini reflectors according tothe invention preferably amounts to between 10 μs and 100 ms, inparticular between 100 μs and 10ms, most preferably around 1 ms. Thetime-of-flight resolution of the ions and their mass resolution increasein line with the passage time. It is also possible to place severalCassini reflectors in series. The diameter and length of a Cassinireflector can be more than 75 and 100 cm respectively.

The following embodiments of Cassini reflectors and time-of-flight massspectrometers represent examples which by no means exhaust the differentdesigns and application possibilities of Cassini reflectors intime-of-flight mass spectrometers. They should therefore not have alimiting effect.

FIG. 6 depicts, by way of example, one embodiment of a time-of-flightmass spectrometer which operates with an orthogonally accelerated ionbeam, as is the case in an OTOF-MS, and uses three Cassini reflectors. Apulser (41) injects a fine ion beam (40), as in conventional OTOF massspectrometers, into a largely field-free flight path (44) and focusesthe new ion beam (43), after a directional correction in the deflectioncapacitor (42), in the usual way onto the entrance slit (45) of thefirst Cassini reflector (46). Ions of the same mass enter the firstCassini reflector temporally focused, with the time distribution widthΔt₁ usual for such pulsers, but also with an energy spread ΔE andangular spreads Δφ_(x) and Δφ_(y). They then fly through the threeCassini reflectors (46), (47) and (48), without increasing the timedistribution width Δt₁, the energy spread ΔE or the angular spreadsΔφ_(x) and Δφ_(y). After exiting the third Cassini reflector, the ionscan then be post-accelerated to 10 to 30 kilovolts, for example in adiaphragm stack (50), reflected with energy focusing in the reflector(51), and measured in the detector (52). This brings about a furthertime distribution width Δt₂ in the non-ideal reflector (52). Anotherpossible option (not shown in FIG. 6) is for the ions to be highlyaccelerated to 10 to 30 kilovolts over a short distance after they haveexited and then impact directly onto a detector.

The time of flight through the reflector, or series of reflectors, canbe several hundred microseconds; with spatially large reflectors(diameter: 150 cm, length: 200 cm) and very low kinetic energies it caneven be milliseconds. This severely limits the repetition rate for themass spectra, and the sensitivity and dynamic measuring range decrease.However, since the high mass resolution means that the mass spectra arelargely empty, a temporal overlapping of the time-of-flight spectra canbe tolerated, and the assignment of the individual time-of-flight peaksto the acceleration pulses of the pulser can be determined from theshape of the peaks, particularly their width, and the shape of theirisotope groups (cf. DE 102 47 895 B4, J. Franzen 2002, corresponding toGB 2 396 957 B or U.S. Pat. No. 6,861,645 B2).

The Cassini reflectors (46), (47) and (48) according to the inventionare of the type depicted in FIG. 5 in a three-dimensionalrepresentation. It corresponds to half a Cassini ion trap according toC. Köster (reference above), with the special feature that the ionsrequire the same oscillation time between injection and ejection in thelongitudinal direction as in the transverse direction. This means thatthe ions can form closed loops, as illustrated in more detail in FIG. 3,when they oscillate in the plane between the inner electrodes. The halfKingdon ion trap is terminated by a plate (38), which is called“equipotential plate” here for reasons of simplicity. Narrow,line-shaped electrodes (39) on the equipotential plate maintain thepotential in the half Kingdon ion trap as it would be present in thefull Kingdon ion trap. The line-shaped electrodes (39) advantageouslyreproduce the equipotential surfaces for this purpose. In addition, theymight be supplied with voltages which correspond to the potentials inthe Kingdon ion trap. The equipotential plate with the line-shapedelectrodes can take the form of an electronic circuit board, forexample, where the resistors which are required as voltage dividers forgenerating the correct voltages are mounted on the rear. The electrodescan also be printed on an insulator, such as a thin ceramic plate, inwhich case it is particularly favorable if the insulator is given a veryhigh-resistance coating before the printing takes place in order toprevent it being charged by scattered ions when it is in operation. Thehigh-resistance coating can even take the form of a voltage divider forthe Cassini potentials.

The back side of the equipotential plate is covered with a singleelectrode plate which is held on the exact potential of the injectionand ejection apertures (36) and (37) respectively. Both aperturesnecessarily are positioned on the same equipotential surface of thereflector. Particularly, the apertures may have the shape of slits, theslits are arranged along an equipotential surface line.

In such a Cassini reflector, ions of the same mass which enter throughthe slit aperture (36) in the equipotential plate (38) with a timesmearing Δt, an energy spread ΔE and lateral angular spreads Δφ_(x) andΔφ_(y), are focused exactly in time t and the lateral angles φ_(x) andφ_(y) onto the exit aperture (37), while maintaining the time smearingΔt, the energy spread ΔE and the lateral angular spreads Δφ_(x) andΔφ_(y).

In the Cassini ion trap, a so-called hyperlogarithmic field is presentwith a potential distribution φ(x, y, z) which is mentioned here for thepurpose of completeness:

${\psi \left( {x,y,z} \right)} = {{{\ln\left\lbrack \frac{\left( {x^{2} + y^{2}} \right)^{2} - {2 \cdot b^{2} \cdot \left( {x^{2} - y^{2}} \right)} + b^{4}}{{ai}^{4}} \right\rbrack} \cdot \frac{U_{l\; n}}{C_{l\; n}}} + {\left\lbrack {{{- \left( {1 - B} \right)} \cdot x^{2}} - {B \cdot y^{2}} + z^{2}} \right\rbrack \cdot \frac{U_{quad}}{C_{quad}}} + U_{off}}$

The shape of the field can be changed by the constants a, b and B.U_(ln), U_(quad) and U_(off) are potential voltages. The inner surfaceof the outer housing and the outer surfaces of the inner electrodes areequipotential surfaces φ(x,y,z)=const. of this potential distribution.

As stated, FIG. 5 depicts half a Cassini ion trap, in which the lateraland the longitudinal oscillation periods are exactly equal for the giveninjection and ejection apertures. It is also possible to set up and useother integer ratios of the oscillation periods. FIG. 7 depicts aCassini reflector based on half a Cassini ion trap in which the ionsexecute precisely one and a half lateral oscillations during half alongitudinal oscillation. When ions are injected here through theentrance aperture (93) in the equipotential plate (95), they are focusedprecisely onto the exit aperture (94) after half an oscillation periodin the parabolic longitudinal field, again with ideal energy and solidangle focusing. Since the penetration depth of the ion trajectories (92)into the parabolic longitudinal field is much greater here than in theembodiment according to FIG. 5, a broader distribution of the ioninjection energy is also possible. The acceptance of a broader, relativespread of energies in turn makes it possible to decrease the averageenergy and thus extend the mass-dispersive time of flight.

FIG. 8 illustrates how such a longer Cassini reflector (72), with theinjection of ions from an RF Paul ion trap, is coupled to atime-of-flight mass spectrometer. In the time-of-flight massspectrometer of FIG. 8, the ions are collected initially in an RFquadrupole Paul ion trap and cooled by a damping gas to form a tinycloud (62). In a first operating mode, which corresponds to that of aconventional three-dimensional RF quadrupole ion trap, the ions can bemass-selectively ejected in the usual way and measured as a massspectrum in a channeltron electron multiplier (66) via an ion electronconverter (67). For many applications, however, this way of acquiringthe mass spectrum does not have a sufficiently high mass resolution andmass accuracy. In a second operating mode, the ions of the ion cloud(62) can also be simultaneously accelerated and pulsed out into anessentially field-free flight path (68), decelerated again in thediaphragm stack (70), and fed to the Cassini reflector (72) at lowenergy with the best possible solid angle and energy focusing at theinjection location (71). The Cassini reflector here is terminated atboth the front and the back with equipotential plates (75) and (74)respectively. As has already been described above, the equipotentialplates (75) and (74) are coated with fine conductive tracks, whichreproduce the Cassini oval of the equipotential surfaces and, whensupplied with the correct voltages, maintain the Cassini potential. Theions leaving the exit aperture (77) are post-accelerated in thediaphragm system (78) with 10 to 30 kilovolts and reflected onto the iondetector (81) in the reflector (80) so as to focus the energy. Thissecond operating mode of the arrangement from FIG. 8 provides a veryhigh mass resolution and a very high mass accuracy, as a high-qualitytime-of-flight mass spectrometer.

The ions do not have to fly through a second reflector (80), however.After a post-acceleration in the diaphragm system (78), they can impactperfectly perpendicularly onto an ion-electron converter plate andrelease secondary electrons there. The electrons are acceleratedbackwards in the diaphragm system (78), re-enter the Cassini reflectorvia the aperture (77), pass through the reflector with their highenergy, leave again through a further aperture (not shown in FIG. 8),and can then be detected in a normal secondary electron multiplier. Thiscombination of a Cassini reflector with a high-quality ion detector hasseveral advantages compared to conventional ion detectors; in particularit causes no additional time smearing of the signals, as occurs inmultichannel plate secondary electron multipliers, for example.

Instead of the RF quadrupole ion trap, a time-of-flight massspectrometer similar to the one shown in FIG. 8 can also be equippedwith a MALDI ion source. The analyte ions are then produced in a plasma,which is generated by laser bombardment of the sample containing theanalyte substance, and are then accelerated with a temporal delay, whichleads to a temporal focusing of ions of the same mass at the entranceaperture (71) of the Cassini reflector of FIG. 8, if the delay time andthe acceleration field strength are set correctly. This arrangementoffers special advantages for the mass spectrometric analysis offragment ions. Ions which decay in the field-free space in front of theCassini reflector are spatially and temporally focused in thisreflector, regardless of the change in kinetic energy compared to themother ion. Additional elements, which are necessary in conventionalreflector systems to accelerate the fragment ions in a special way, arenot required.

It is also possible to build Cassini reflectors which are even slimmerand which penetrate to greater depths into the parabolic potential inthe longitudinal direction. The ions may then execute 5/2, 7/2 or 9/2transverse oscillations per half a longitudinal oscillation. Thisincreases the acceptance for ions with a broad relative energy spread.

Furthermore, it is not necessary to inject the ions in the center planeof the Cassini reflector in order for them to be ideally reflected. FIG.9 shows how an ion beam enters away from the center plane, and alsoexits again away from the center plane, ideally focused according toenergy and solid angle. This behavior can also be used for a doublepassage of a Cassini reflector. FIG. 10 shows such an arrangement. Ifthe penetration depth of the ion beam into the first Cassini reflectoris around one meter, the entrance beam (105) and exit beam (106) canquite easily be around six centimeters apart. It is thus possible tomake heavy ions with a molecular weight of 3000 daltons from an RFquadrupole ion trap pass through a mass-dispersive time of flight of afew milliseconds. The time-of-flight spectrum can be measured with 400million measurements per second using a 16-bit ADC, and producesresolutions of R>100 000 in the medium mass range.

The housing of the Cassini reflectors according to FIGS. 5 to 7 is notvery easy to manufacture. Additionally, the interior of the largelyclosed Cassini reflectors is not easy to evacuate. FIG. 11 thereforeshows a Cassini reflector of a completely different embodiment, but withthe same electric field. The outer housing here is replaced by a stackof identical apertured diaphragms (122), as are used in a similar designaccording to the prior art for Mamyrin reflectors. The apertureddiaphragms here have inner openings in the form of a Cassini oval,however. In order to maintain the electric field of a Cassini ion trap,the apertured diaphragms are supplied with a quadratically increasingpotential from the direction of the equipotential plate (120). Theequipotential plates (120) and (121) correspond to those in FIG. 7. Ionsof different energies fly on trajectories (124) which extend todifferent depths into the reflector, but all have precisely the sametime of flight for ions of the same mass. This embodiment has severaladvantages: the reflector is easier to evacuate; the overall size issmaller, the manufacture is simpler and lower cost.

It shall be mentioned that the inner electrodes can also be assembled asstacks of identical diaphragms, which may be supplied with aquadratically decreasing potential. The manufacture is possibly morecomplicated than the manufacture of compact inner electrodes, however.

The person skilled in the art will find it easy to develop furtherinteresting embodiments based on the devices for the reflection of ionsaccording to the invention. The part which is subject to this inventionshall also be covered by this patent protection application.

1. (canceled)
 2. A time-of-flight mass spectrometer having an ionsource, a flight path, a single reflector with the potentialdistribution of a Cassini ion trap within the flight path, and an iondetector, wherein the single reflector is one halved Cassini ion trapwith a housing, several inner electrodes and a terminating equipotentialplate with electrodes, the electrodes of the equipotential plate tracingthe equipotential surfaces of the potential distribution of the Cassiniion trap at the location of the equipotential plate.
 3. Thetime-of-flight mass spectrometer according to claim 2, wherein theequipotential plate has apertures for the injection and ejection ofions.
 4. The time-of-flight mass spectrometer according to claim 3,wherein the shape of the reflector and the positions of the injectionand ejection apertures are designed so that ions of the same mass passthrough an odd whole number of transverse half oscillations in thereflector during a half longitudinal oscillation.
 5. The time-of-flightmass spectrometer according to claim 2, wherein the housing of thereflector is constructed as a stack of identical apertured diaphragms,with a voltage supply which generates a potential that increasesquadratically from diaphragm to diaphragm.
 6. The time-of-flight massspectrometer according to claim 2, wherein at least one diaphragm systemis present accelerating or decelerating the ions in such a way that theypass through a reflector with a kinetic energy of less than twokiloelectronvolts.
 7. The time-of-flight mass spectrometer according toclaim 2, wherein the time-of-flight mass spectrometer includes a pulserfor the orthogonal injection of a fine ion beam.
 8. The time-of-flightmass spectrometer according to claim 2, wherein the time-of-flight massspectrometer includes an RF quadrupole ion trap.
 9. The time-of-flightmass spectrometer according to claim 2, wherein an ion accelerationsystem with a conversion plate is mounted at the exit of the reflector;the conversion plate converting the ions into electrons, which then flybackwards through the reflector with a high energy; and a secondaryelectron multiplier for detecting the electrons is mounted behind therear equipotential plate.
 10. A time-of-flight mass spectrometer havingan ion source, a flight path, multiple reflectors within the flightpath, and an ion detector, wherein each reflector comprises one halvedCassini ion trap with a housing, several inner electrodes and aterminating equipotential plate comprising an injection aperture, anelection aperture and electrodes, the electrodes of the equipotentialplate tracing the equipotential surfaces of the potential distributionof the Cassini ion trap at the location of the equipotential plate, andwherein the halved Cassini traps are shifted to each other with regardto the longitudinal direction such that the ejection aperture of apreceding reflector is aligned to the injection aperture of a subsequentreflector.
 11. The time-of-flight mass spectrometer according to claim3, wherein the injection and ejection apertures have the shape of slits.12. A time-of-flight mass spectrometer having an ion source, a flightpath, a reflector inside the flight path and an ion detector, whereinthe reflector is a Cassini ion trap with a first and a second housingand two inner electrodes, the second housing being smaller than thefirst housing and supplied with a lower voltage difference to the innerelectrodes than the first housing so that the electric fields in theinterior of the Cassini ion trap are maintained, and wherein thereflector comprises an ion injection point and ion exit point, thepoints being at the interface of the two housings such that ions travelfor a half longitudinal oscillation in the interior of the first housingand are transferred from the injection point to the exit point.